The present invention relates to devices and methods for testing tack uniformity of a coating on a substrate. The term "coating", as used herein, is intended to refer to any liquid that is applied to a substrate, including both inks and non-ink coatings. Substrates include coated and non-coated papers, and other coated and non-coated sheet materials.
Tack is one of the most important characteristics of coatings, affecting, for example, the runnability and printability of papers. Tack does not remain constant after a coating is applied, but instead changes over time as the coating consolidates, interacts with the paper surface, and dries. The change in tack as a function of time is generally referred to as "tack build-up". In multi-color offset printing, tack build-up of inks is important both to ink-setting and to the transfer of additional ink film to the fresh ink layers already on the paper. Part of the fresh ink already on the paper may retransfer or "backtrap" to subsequent printing blankets. If the tack build-up is not uniform over the paper surface, the local transfer/retransfer balance is upset and the resulting print will not be uniform and may appear mottled. This partial or uneven setting of the ink on a microscale is referred to as "backtrap mottle" (BTM), and can result in an unacceptable blotchy or mottled printed surface with alternating light and dark printed areas. The size of the light and dark areas varying in print density are a distinguishing characteristic of a given printed substrate. Generally, BTM is more noticeable and objectionable for larger printed areas, smaller halftones, coarser BTM patterns, and larger spacings of the light and dark areas. BTM can be measured by commercially available instruments or by subjective expert evaluation.
There is often a need in the paper-making, coating and printing industries to measure tack properties, such as tack build-up, so that BTM and other production problems can be predicted and minimized. As a result, a number of devices have been developed to measure tack build-up. Typically, these devices integrate over a large area, so that the presence of small-scale (less than 20 mm.sup.2) areas of non-uniform (slower or faster) tack build-up may not be detected.